Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Don Brady <dev.fs.zfs@gmail.com>
Reviewed by: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Author: Chris Williamson <Chris.Williamson@delphix.com>
illumos/illumos-gate@89c86e3229
Currently, every buffer cached in the L2ARC is accompanied by a 240-byte
header in memory, leading to very high memory consumption when using very
large cache devices. These changes significantly reduce this overhead.
Currently:
L1-only header = 176 bytes
L1 + L2 or L2-only header = 176 bytes + 32 byte checksum + 32 byte l2hdr
= 240 bytes
Memory-optimized:
L1-only header = 176 bytes
L1 + L2 header = 176 bytes + 32 byte checksum = 208 bytes
L2-only header = 96 bytes + 32 byte checksum = 128 bytes
So overall:
Trunk Optimized
+-----------------+
L1-only | 176 B | 176 B | (same)
+-----------------+
L1 & L2 | 240 B | 208 B | (saved 32 bytes)
+-----------------+
L2-only | 240 B | 128 B | (saved 116 bytes)
+-----------------+
For an average blocksize of 8KB, this means that for the L2ARC, the ratio
of metadata to data has gone down from about 2.92% to 1.56%. For a
'storage optimized' EC2 instance with 1600GB of SSD and 60GB of RAM, this
means that we expect a completely full L2ARC to use (1600 GB * 0.0156) /
60GB = 41% of the available memory, down from 78%.
c908dc6f4b